The present invention relates to a short-cycle arc welding system for welding elements, such as metal studs, to components, such as metal sheets, having:                a robot that has at least one arm that can move in at least one coordinate axis        a welding head that is mounted on the arm and on which is provided a holder for holding an element and a linear motion device for advancing and retracting the holder relative to the welding head, and        a measurement system for determining the relative position of a component to an element to be welded to the component and held by the holder, wherein the measurement system has a foot mounted on the welding head that is designed to contact the component during operation in order to determine the relative position of the element to the component.        
The present invention also relates to a corresponding method for short-cycle arc welding. Such a short-cycle arc welding system and an associated method are generally known. In short-cycle arc welding, an element is welded onto a component. In this process, an arc that melts the end faces is formed between the element and the component. The element and component are then moved toward one another so that the molten materials combine. The arc is short-circuited and all the molten material solidifies. It is customary for the arc to be drawn. In this process, the element is first placed in contact with the component. A pilot current is then switched on and the element is raised to a desired height above the component, drawing an arc. Only then is the welding current switched on.
To achieve consistently good welding results, it is important to know the relative positions of the element and component, and in particular to raise the element to the correct height before the welding current is switched on. To this end, a measurement of the relative position is generally performed before each welding operation, particularly in the form of a determination of the zero point. This is especially important in robot-based systems. It is true that modern robots generally are able to position relatively precisely. However, due to the large moving masses, high precision cannot be achieved with very high speeds. The robots customarily have a range of motion in three coordinates. In the simplest case, a robot is an automatically operated linear guide (carriage) on which is mounted a welding head.
Stud welding systems are used in the automotive industry in particular, where they are used primarily to weld elements such as threaded and unthreaded studs, eyes, nuts, etc. to the car body sheet metal. These elements are then used as anchors for attaching such items as interior paneling and trim. Speed of production is a critical factor in the automotive industry. Hundreds of elements must be automatically welded to different positions by robots within a space of just a few minutes. Consequently, the robots must be moved at high speeds. Accordingly, it is known to affix to the arm of a robot a welding head base that supports a carriage. The carriage is capable of moving at high speeds with high precision, typically by means of a pneumatic or hydraulic system. Mounted on the carriage is the actual welding head, which in turn has a linear motion device for moving the element.
It is known to affix a so-called support foot to the welding head in order to determine the relative position of the element to the component (for example, from the publication “Neue TUCKER-Technologie. Bolzenschweiβen mit System,” Emhart Tucker, September 1999). The support foot is aligned approximately parallel to the welding head holder. In an initial position, the element held in the holder projects somewhat beyond the support foot. In order to determine the relative position, the welding head is made to approach the component. During this process, the element contacts the component first. The welding head is advanced further until the support foot contacts the component. In this process, the holder is displaced relative to the welding head in opposition to an elastic preloading force. As a result, the relative position of the element to the component can be determined by means of a suitable measurement system and the rigid connection between the support foot and the welding head.
Alternatively, so-called footless measurement systems are also known for determining the relative position of the element to component. Thus, U.S. Pat. No. 5,252,802 discloses a stud welding apparatus with a housing designed as a gun. A positioning motor first brings the housing into a position in which a stud is located in the vicinity of a component. Provided in the housing is a linear motor for the purpose of axially moving a shaft that carries the stud. A position measurement system is provided to control the linear motor. To determine the relative position of the stud and the workpiece, the linear motor is actuated so as to move the stud toward the workpiece at a specific speed. As soon as the stud touches the workpiece, an electrical contact closes. In addition, it is known from WO 96/11767 to elastically preload the stud holder in the direction of the workpiece and to move it axially against the preloading by means of a linear motor. Finally, WO 96/05015 discloses a stud welding apparatus with no support foot, wherein a welding head can be moved as a whole by means of a positioning drive. Provided on the welding head is a holder that holds a stud. A positioning device serves to move the holder axially relative to the welding head. The positioning device can be a servo-pneumatic or servo-hydraulic working cylinder. The relative position of the holder to the welding head is determined by means of a position measurement system.
In order to determine a zero position between the stud and the workpiece, the welding head is moved to an end position in the direction of the workpiece. In the course of this movement, the stud contacts the workpiece. Because from this point on the stud can no longer follow the motion of the welding head, the holder is then displaced relative to the welding head in opposition to the pressing motion. This displacement is measured by the position measurement system with the result that the final position of the welding head is determined precisely.
With the above in mind, an object of the present invention is to specify an improved short-cycle arc welding system and method for short-cycle arc welding. This object is achieved in the aforementioned short-cycle arc welding system in that the measurement system is additionally designed to determine the relative position of the element to the component without any contact between the foot and the component, and in that the measurement system has means to move the foot from an operating position to a rest position in which the foot is inactive.
In the aforementioned short-cycle arc welding method, the object is achieved in that the method uses a short-cycle arc welding system that has a control unit in which is stored, for a plurality of automatic welding processes, information on whether determination of the relative position of the element to the component should take place in a given welding process with or without a foot that is designed to contact the component, and further wherein the method has the following steps:                a) operating a robot with an arm such that a welding head attached to the arm reaches a welding position for a selected welding process,        b) establishing whether the selected welding process requires the relative position of the element to the component to be determined with or without the foot,        c) determining the relative position of the element to the component as a function of the conclusion reached in step b), and        d) carrying out the selected welding process.        
With the invention, it is possible to determine the relative position of the element to the component with or without a foot or support foot. Generally, the relative position is determined without a support foot in weld locations where the component is relatively stable. This has the advantage that the welding stud can be welded especially close to contours since more space is available that is not occupied by a support foot. The higher stiffness in corners and at folds etc. also makes it possible to do without a support foot.
In contrast, when a welding process is to be carried out on a relatively unstable component, for example unsupported thin sheet metal, it is preferred to determine the relative position of the element to the component with a support foot. In such a case, the foot provides a type of “bracing” of the component relative to the welding head. As a result, the component cannot deflect. In such a case, the relative instability of the component could result in incorrect measurements if the relative position is determined without using the support foot. The object is attained in full in this way.
It is particularly advantageous for the measurement system to be designed to determine the relative position of the element to the component without contact between the foot and the component in that the point when the element contacts the component is measured when the element approaches the component. In general, this embodiment permits especially rapid determination of the relative position. In this context, it is particularly advantageous for the contact between the element and the component to be measured electrically. This measurement can, for example, be accomplished in that the rise of the motor current in an electric motor that is used to move the element toward the component is measured. Alternatively, a voltage could also be applied between the element and the component. The collapse of this voltage to zero then indicates that the element has contacted the component electrically and consequently also mechanically.
In an alternative embodiment, the contact between the element and the component is measured in that, while the welding head approaches the component, the element is displaced relative to the welding head after contact with the component and the relative position of the element to the welding head is measured. This embodiment corresponds to the method disclosed in the aforementioned WO 96/05015. It is advantageous overall for a control unit to be provided in which is stored, for a plurality of automatic welding processes, information on whether determination of the relative position of the element to the component should take place with or without a foot in each welding process. In this way, determination of the relative position with or without the foot can be performed on a case-by-case basis when a plurality of welding processes are performed one after another by a robot. Prior to each welding process, the foot is moved, either to its operating position or to its rest position. This can even take place during movement of the welding head from one weld point to a next weld point by the robot, for example.
In another preferred embodiment, the welding head has elastic means for preloading the holder in an actuating direction. As a result, the linear motion device for advancing and retracting the holder can be left de-energized in most operating states. The result is low energy consumption. In accordance with a preferred embodiment, the elastic means preload the holder in the retracting direction. When the linear motion device is operated in the advancing direction in order to determine the relative position of the component to the element without a foot, the holder in its elastically preloaded rest position is always in the correct initial position. Thus, especially low energy consumption is achieved on the whole. Moreover, under certain conditions, higher speeds can be achieved than in other embodiments.
In an alternative embodiment, the elastic means preload the holder in the advancing direction. In this embodiment, a higher speed can be achieved in the advancing direction in the actual welding process. In the event of preloading in the retracting direction, the holder must be extended against the preloading during determination of the relative position with the support foot, either before or after the support foot has mechanically contacted the component. With preloading in the advancing direction, active movement of the holder by means of the linear motion device is not absolutely necessary when determining the relative position using the support foot.
In another preferred embodiment, the measurement system has a position sensor that measures the position of the holder relative to the welding head. Of course, the features mentioned above and those explained below can be used not only in the combinations specified, but also in other combinations or by themselves without exceeding the scope of the present invention.